October 17, 2012

Forest Corridors Key To Saving Sumatran Orangutan Genetic Diversity

The Sumatran orangutan population is in decline, according to a new genetic study. However, researchers have identified critical corridors for dispersal migrations. If these migrations are protected, they can help maintain genetic diversity to aid in the species' conservation.

The Sumatran is one of two species of orangutan, and is designated as "critically endangered" by the IUCN Red List. The orangutan once roamed the entire island of Sumatra, but now only numbers an estimated 6,600 individuals and is restricted to small forest patches on the northern tip of the island. The main reason for this decline in both numbers and range is large-scale deforestation.

This is best illustrated by the story of Seuneam, a Sumatran orangutan recently rescued from an isolated forest area where palm oil companies have been illegally destroying the environment.

Seuneam was trapped for several days in an area surrounded by palm oil plantations, isolated from the rest of the ape population in the Tripa Swamp area of the Nagan Raya district. The Sumatran Orangutan Conservation Program successfully found and rescued the male. The Tripa area is legally protected, but many palm oil companies are under investigation and one plantation had its permit cancelled.

A new study, published in the Journal of Heredity, examined the Sumatran orangutan for population structure, movement patterns, and reproductive interchange using genetic techniques. The research team took fecal and hair samples from orangutans throughout their Sumatran range to isolate DNA. They also took blood samples from orangutans that had been kept privately as pets before being confiscated by authorities. Using two different genetic markers - mitochondrial DNA, which is inherited only from an individual's mother, and autosomal microsatellites, short, repeated DNA elements that are inherited from both parents — the team looked for population structure and gene flow.

"The orangutans from one of the study areas on the west coast of the island exhibited very high genetic diversity," explained Dr. Alexander Nater of the University of Zurich Anthropological Institute & Museum. "This diversity is a clear indication of a large historical population size. However this area currently harbors only around 400 orangutans," leading the authors to conclude that the population has recently declined dramatically.

The study findings revealed that geographical barriers, including major rivers and a large volcanic caldera, have created a number of subpopulations within the pronounced population structure of the Sumatran orangutans. According to Huffington Post´s Our Amazing Planet, the barriers have isolated groups of the apes, some of which contain only a few hundred individuals.

"Such isolated, small populations will inevitably suffer from a decline in genetic diversity and negative effects of inbreeding," said Nater. "This means that local orangutan populations are at substantial risk of extinction."

When a subpopulation adapts to a specific local environmental factor, such as a food source or disease, the extinction risk can be exacerbated. The local environment allows the group to survive in the short-term, but if the environment changes quickly, the group will be unable to adapt.

One element that the research team uncovered gives them hope. There is genetic evidence for recent reproductive interchange between the groups, specifically by breeding males. This genetic exchange is absolutely necessary for the health of the entire Sumatran orangutan population.

"Our study revealed that some males can range widely over large distances and across natural barriers in search of females," Nater said. “By doing so, they are killing two birds with one stone. On the one hand, they avoid the conflict with the dominant local males and thus increase their chances of breeding successfully; at the same time, however, they also reduce the risk of mating with closely related females from their place of birth.”

This led the researchers to speculate that the distinct dominance structure of Sumatran orangutans constitutes a natural mechanism to guarantee the genetic exchange that is necessary to offer hope of survival to this species of great ape.

Examining the data revealed a specific inland high-elevation area as an important reproductive interchange corridor across the island. The breeding males use this passage to circumvent the geographical barriers to provide important genetic exchange among the different populations.

Sumatran orangutans are the most arboreal of the great apes, spending nearly all their time in the tree tops, making it critical that this corridor remain forested.

"This result highlights the need to conserve these important dispersal corridors to uphold genetic exchange," Nater said, "and it also gives hope that it is not yet too late to preserve these unique Asian great apes."

To accomplish this goal, a change in conservation needs to occur. Currently, conservation efforts focus on the peat bog forests on the northwest coast. The highest concentration of orangutans live there, and there is a sizeable interest in economic development. This study shows that the rainforest corridor needs to be the focus of future efforts.

“While these mountain forests are not home to any viable orangutan populations, their value for the protection of the species should by no means be underestimated as the roaming orangutan males traverse these habitats on the lookout for the next population and thus preserve the genetic diversity. These mountain regions should therefore take on a key role in the strategy to protect the Sumatran orangutans,” concludes anthropologist Carel van Schaik.